1. Field of the Invention
The present invention relates to an optical proximity sensor according to the confocal optical imaging principle for determining distance or height values to or from a surface of an object to be measured. The plane coordinates of the multiplicity of measurement points are specified by the lateral displacement of a proximity sensor relative to the object to be measured, a grid pattern being generated.
2. Description of the Prior Art
Preferred areas of use of the present invention are in the field of connection and mounting processes in the fabrication of semiconductors and printed circuit board assemblies. As a rule, objects to be inspected have a three-dimensional character; for example, in the shape and height of the soldering bumps in ball grid arrays, in flip-chip contact-making or in the shape of nailheads in wire bonding. For this reason, fast three-dimensional sensing of the regions to be checked is necessary. As a rule, however, conventional three-dimensional sensors do got satisfy the requirements with regard to acceptable costs and adequate detection reliability in the case of highly reflective, curved areas.
Not only electrical testing but also external, mainly optical inspection has a significant role to play in the context of quality inspection in the fabrication of electronic parts. Since ever higher requirements with regard to quality are made of the connection and mounting processes in the fabrication of semiconductors and printed circuit board assemblies, the inspection methods that are correspondingly employed must be refined to an increasing extent. Thus, defect rates of less than 10 dpm (defects per 10.sup.6) are required. This ambitious aim cannot be achieved merely by optimizing the processes. Ultimately, automatic inspection is required after virtually every process step.
As a rule, conventional two-dimensional image processing methods do not satisfy today's requirements.
Triangulation methods are also frequently employed for the three-dimensional sensing of surfaces. However, at a resolution of 10 .mu.m, for example, these methods are considerably restricted for optical reasons, such as secondary light reflection, for example. Confocal systems with a coaxial beam arrangement are significantly better suited for this purpose. However, the data rate that can be achieved is very limited as a result of the mechanically moved objective.
European Patent Specification EP 0 615 607, which relates to an optical proximity sensor, describes how to increase the data rate to 2 MHz and how to effect areal sensing using the confocal principle. The high data rate is achieved by a scanning system based on fast beam deflection. The relative movement between the object to be inspected and the scanning sensor is effected in a meandering manner. In order to avoid mechanical movements of the objective, a height determination is carried out on the image side by means of detectors which are staggered axially on the measurement beam. However, this principle, which is highly developed in terms of technology, is associated with very high costs.
Confocal inspection methods which, depending on the design, are largely independent of the optical properties of the surfaces are in any event being employed to an increasing extent in three-dimensional surface measurement. This is ideally achieved by subjecting the object surface to point illumination and by imaging the reflected light in turn on a point detector. The smaller the illuminated area and the detector area corresponding thereto, the greater the resolution and the higher the level of suppression of interference due to secondary scattered light and incorrect measurements due to the shape or due to the curvature in the region of the measurement spot. This can be formulated mathematically in such a way that even greatly curved, reflective areas appear to be flat when the measurement spot is a great deal smaller than the radius of curvature of the area.
WO-A-92/14118 describes an optical sensor which functions with structured light. The light source is adjustable here in such a way that it is possible to implement a change of the positions of high-contrast regions in a pattern, wherein an arrangement of detector elements form the detector. In this case, two images are each focused on an object, the focusing of the first image on the object being varied.
U.S. Pat. No. 4,719,341 discloses a so-called proximity sensor. In this case, the optical path length is varied but an intermediate image is not generated at any point. Converging light pencils do not pass to adjustable mirrors, with the result being that the requirements of a confocal system (point light source, point detector, object focused) are not given.
Conventional, commercially available confocal microscopes indeed have these optical properties but, as a rule, are too slow for the use in fabrication in light of the requisite mechanical tracking of the objects and/or of the microscope objectives.